1. Field of the Invention
The invention relates to a grinding element, which can be arranged on the outer surface of a frame of a grinder stone used to defiber wood, the grinding element comprising at least one grinding segment made of grinding ceramic material, and fastening means for fastening the element.
The invention further relates to a grinder stone for defibering wood, comprising a cylindrical frame, grinding segments made of grinding ceramic material and arranged on the outer circumference of the grinder stone, and a shaft, around which the grinder stone is arranged to be rotated during grinding.
2. Description of Related Art
Wood is typically ground into fibers by means of grinding machines, where logs are pressed against the surface of a rotating grinder stone. Water is simultaneously sprayed to purify and cool the grinder stone. The stone makes the wood fiber matrix vibrate, whereafter the wood fibers are released from the logs to provide a pulp suspension.
The frame of the grinder stone is usually made of concrete, but steel frames have also been developed. A grinding surface of the grinder stone consists of separate ceramic grinding segments attached to the frame. The grinding segments are attached by various mechanisms, which press each segment separately against the outer circumference of the grinder stone. During use, the grinding segments are subjected to a force, which acts parallel to the tangent of the grinder stone and tends to detach the segments. Furthermore, rotating the grinder stone subjects each segment to a centrifugal force, wherefore the segments must be firmly secured.
Due to different thermal expansion coefficients of the ceramic segment, the frame of the grinder stone, and the fastening bolts, the prior art teaches fastening bolts, which can be used to compensate for varying dimensions resulting from changes in the temperature. However, such bolts are rather complicated and therefore also expensive. Furthermore, it is possible to place semi-spherical washers between the bolt and the segment in order that the stresses resulting from the fastening will be distributed more evenly in the ceramic segment and the segment will thus be able to withstand the fastening forces. Another problem with the present arrangements is that the fastening forces subject the ceramic segment to compression stress, which can break the segment usually from below the washer. It is thus necessary to restrict the fastening force, which in some cases can lead to insufficient fastening of the segment. During grinding, such a loose segment starts vibrating and will be damaged. The ceramic materials used in grinding are highly wear-resistant and hard, yet brittle. However, in practice grinding segments cannot be manufactured from more resistant ceramic materials, since the ceramic material used in grinding of wood must be of a particular type and sufficiently porous to provide a desired grinding effect. Also, quality characteristics set for mechanical pulp require use of the present grinding ceramic materials.
Yet another problem with the present grinding segments is that replacing the segments is difficult and slow.